Heads with three necks (inline) that fit our 6504,6511&6516 reaction vessels that feature either a 137 x 100mmID or 168 x 130mmID flat flange. Use with our 6508 or 6510 clamps. For improved sealing of flat ground flanges, please see our 6495 product family of gaskets. Note: vessels with o-ring grooves seal with just the o-ring supplied with the vessel.